A conventional example of a taphole structure of a melting furnace used for melting ores or the like, or a melting furnace such as a cupola is disclosed in Japanese Unexamined Utility Model Application, First Publication No. S62-194748. In the conventional example, a fire-resistant ramming material, which is a gap filling material, is filled into a hole provided in a furnace wall brick, and a tapping hole is opened to serve as a taphole. Also, a structure made by setting an iron pipe in the hole and burying the vicinity thereof with a fire-resistant ramming material, a structure made by preparing a taphole-attached precast member and mounting this in the hole of the furnace wall brick, and the like are known.
A taphole is a passage for flowing hot metal, which is subjected to melting in a melting furnace, out of the furnace. The vicinity of the taphole and an inner peripheral surface thereof always come into contact with high-temperature hot metal. Accordingly, there is a problem in that wear due to the hot metal and melting loss always occurs, thus the life of the taphole is very short. Particularly, in case of a taphole made of an iron pipe, wear is high. In addition, there is a problem in that when the diameter of the taphole expands due to the melting loss or the like in the taphole and the peripheral portion, there is a concern that gas in the melting furnace will be ejected from the furnace along with the hot metal.
Therefore, a replacement of the taphole is required every five or six days. In addition, the inner peripheral surface of the taphole needs to be repaired about every ten days, and frequent repair work is necessary.
For repair work on the taphole, generally, a hot pouring method of flowing alumina and silicon carbide ceramics into a worn point along with water, and a precast method of replacing the vicinity of the taphole with a tapping hole-attached precast member which is individually molded are employed.
Particularly, the precast method has problems in that as a dismantled part becomes larger, a longer period of repair is needed, resulting in a further increase in cost. Accordingly, the hot pouring method without the above-mentioned problems has been generally used.
However, in the hot pouring method, there is a problem in that it is difficult to perform a complete repair on the vicinity of the inner peripheral surface of the furnace, and durability is low even after the repair, so that frequent repairs are needed.
FIGS. 5A to 5D schematically show a repair method using the hot pouring method. FIG. 5A shows an initial state, FIG. 5B shows initial damage, FIG. 5C shows a state after a first repair, and FIG. 5D shows a state after a second repair. In FIGS. 5A to 5D, reference numeral 21 denotes a furnace wall brick, reference numeral 21a denotes a tapping hole provided in the furnace wall brick 21, reference numeral 22 denotes a pipe made of iron, which is disposed inside the tapping hole 21a, and reference numeral 23 denotes a ramming material filled into a gap between the pipe and the tapping hole 21a. 
In the hot pouring method, as shown in FIG. 5B, in a case where damage of the ramming material 23 occurs, a metal frame 24 for hot pouring is set outside of the furnace as shown in FIG. 5C, and a pouring material 25 made of alumina and silicon carbide ceramics is injected for repair. However, since gas pressure is applied from inside the furnace and scattered residuals remain after dismantling, the pouring material 25 cannot reach the inner side, and, as a result, a space 26 is formed. Therefore, a complete restoration cannot be achieved. Furthermore, as shown in FIG. 5D, although the space 26 gradually increases, this cannot be prevented. As such, in the repair method using the hot pouring method, there is low durability and frequent repairing work is necessary. Therefore, the development of a new taphole structure and a repair method thereof capable of increasing the life of the taphole is required.